Time to do a leak down test I would say:-
http://en.wikipedia.org/wiki/Leak-down_tester
You're going to need a compressor and some adapters (usually supplied in a leak down test kit) to apply low compressed air to each cylinder in turn via the glow plug holes.
Even if you can't get your hands on a tester you can still apply low pressure to each cylinder (when the valves are shut) to determine where things leak the most => exhaust pipe (exhaust valve problem), air filter (inlet valve), oil filler cap / dip stick tube (could be leaking past pistons or cracked head - remove valve cover to check).
This will help with the smoking problem...
But for the over heating problem - What is happening to the oil pressure when it gets so hot?